When spaced-apart conductors, such as the conductors in a flat cable, must be soldered to spaced-apart terminal pads on a substrate, such as the terminal pads on a circuit board, it is common practice to carry out the operation by means of an apparatus having a rigid resistance element, commonly referred to as a heater bar, which is heated by passing a current therethrough to cause the bar to be heated by the I.sup.2 R heating effect. In the use of apparatus of this type, the conductors are positioned against the terminal pads by means of a suitable fixture or work holder and the bar is brought into engagement with the conductors so that the solder, which was previously applied to either the conductors or the terminal pads, will be melted and will bond the conductors to the pads. While apparatus of this type can be used under many circumstances, problems are sometimes encountered for a number of reasons For example, the heating bar is heated along its entire length and the heating is controlled usually by a single thermostat placed at some location along the bar. If one portion of the bar is cooled, as by an air current, to a greater extent than another portion, the portion which is cooled will be at a lower temperature than the remainder of the bar and soldering of the conductors in contact with the low temperature portion of the bar may not be satisfactorily carried out. Also, the fact that the heater bar must be heated along its entire length sometimes gives rise to a problem of overheating in that heat is required only at the actual sites where the soldering operation is being carried out, that is, where the wires are contacted by the soldering bar, and the portions of the soldering bar between adjacent wires may be heated to a higher temperature than those portions which are in contact with the wires. If the substrate on which the terminal pads are located is incapable of withstanding the heating effect of the intervening portions of the bar, the substrate may be damaged by radiant heat from the heater bar. The fact that the heater bar is a rigid member can also cause problems in that it will be positioned against the conductors and if one or more of the conductors is slightly undersized, good physical contact between the heater bar and the undersized conductor may not be achieved with a resulting reduction in the amount of heat which is transferred to the undersized conductor so that it may not be properly soldered to its associated terminal pad. Finally, it should be mentioned that under ideal conditions, the heater bar contacts the conductors only along a short line or at a point of tangency and heat transfer can take place only at this single point or line. It would be desirable to increase the area of contact of the heater element with the individual conductors thereby to improve the heat transfer capabilities at the interface of the heater and the conductors.
The present invention is directed to the achievement of an improved heater bar assembly and an improved apparatus which overcomes or minimizes the problems discussed above and which presents other advantages as will be discussed below. The improved results achieved in the practice of the present invention result from the use of a bi-metallic self-regulating Curie effect heater which is supported in a heater bar assembly in a particular manner and which is incorporated into the soldering apparatus. The term "bi-metallic self-regulating Curie effect heater" as used herein is intended to refer to a type of heating device described in U.S. Pat. No. 4,256,945 and in U.S. Pat. No. 4,695,713. Heaters of this type are made up of a magnetic material and a non-magnetic material, the magnetic material having a predetermined Curie point. When a high frequency current is passed through the heater, the current is concentrated as a result of the skin effect in the magnetic portion thereof and it is rapidly heated to its Curie temperature at which the magnetic permeability of the material abruptly falls so that the temperature does not rise significantly above the Curie temperature. If heat is extracted from the system, the temperature may drop below the Curie temperature and the magnetic permeability of the magnetic layer will increase to its higher level. The rate at which heat is generated will then increase as a result of the skin effect so that the heater is self-regulating. Reference is made to the above-identified U.S. Patents, which are hereby incorporated by reference, for further details of the construction and operation of Curie-type heaters.